Colour stabilization of base-stabilized ethylenically modified unsaturated monomers

ABSTRACT

The invention relates to the use of at least one compound from the tocopherol group for the colour stabilization of ethylenically unsaturated monomers, particularly hydroxyalkyl(meth)acrylates, which already comprise at least one polymerization inhibitor for base stabilization or storage stabilization. Such colour-stabilized and polymerization-inhibited monomers are preferably used in clearcoat and high solid paints.

[0001] Color stabilization of base-level-stabilized ethylenicallyunsaturated monomers, in particular of base-level-stabilizedhydroxyalkyl (meth)acrylates.

[0002] The invention is situated within the field of the stabilizationof polymerizable compounds, referred to as monomers. The inventionrelates in particular to the color stabilization of ethylenicallyunsaturated monomers, in particular of hydroxyalkyl (meth)acrylates,which already have at least one polymerization inhibitor for base-levelstabilization or storage stabilization.

[0003] Ethylenically unsaturated monomers tend more or less towardspontaneous polymerization. The polymerization reaction is a chainreaction which in general is triggered by free radicals. The formationof these radicals can be promoted by light and/or the effect oftemperature. In order to prevent spontaneous polymerization, monomersare stabilized both during their preparation and during storage byaddition of polymerization inhibitors. The number of polymerizationinhibitors for the base-level or storage stabilization of ethylenicallyunsaturated monomers is legion.

[0004] The unwanted spontaneous polymerization of ethylenicallyunsaturated monomers is manifested for example in an increase in theviscosity of the monomer, which is frequently liquid under standardconditions of pressure and temperature (atmospheric pressure and ambienttemperature). Whether a liquid monomer contains polymeric fractions orwhether this fraction of polymeric constituents changes, i.e., usuallyincreases, over the course of a storage period, is something which canbe determined with great exactitude by means for example ofnephelometric measurements.

[0005] For many applications, however, it is not only a spontaneouspremature polymerization of the monomers that is problematic: the colorof the monomers as well is of interest. Coating materials frequentlyinclude free-radically curing (polymerizable) monomers. In the case ofclearcoat materials and high-solids coating materials particularimportance is placed on their being free from intrinsic color, i.e.,exhibiting no discolorations. As a consequence, neither mustdiscolorations be carried over into the coating compositions by way ofmonomers which already have a tinge of color, nor must additionaldiscolorations or tinges of color develop during the storage of thecoating composition prior to its actual application. The result of bothis that the clearcoat and high-solids coating materials are unusable.

[0006] Particularly in the case of hydroxyalkyl (meth)acrylates it ispossible during storage, before the coating material is blended, toobserve a yellow coloration which is prohibitive for the use of themonomer in such applications, for which colorlessness (absence ofintrinsic color) is a prerequisite.

[0007] There accordingly exists a particular need for storage-stable(nonpolymerizing) and at the same time color-stable (nondiscoloring)ethylenically unsaturated monomers, especially hydroxyalkyl(meth)acrylates, for manifold applications, particularly asfree-radically curing constituents in coating materials.

[0008] As regards the specific state of the art the followingpublications are cited:

[0009] WO 99/01410 (Betzdearborn Inc.)=D1;

[0010] WO 90/07483 (Henkel KGaA)=D2;

[0011] Derwent Abstract: JP 58201725 A (EISAI Co. Ltd.)=D3;

[0012] WO 99/48997 (CIBA Specialty Chemical Holding Inc.)=D4; and

[0013] DE 195 21 848 A1 (BASF AG)=D5.

[0014] D1 deals with the inhibition of the polymerization of monomers.D1 discloses in particular a method of preventing the polymerization ofvinyl monomers during preparation, storage, and transit. D1 shows inparticular that alpha-tocopherol is a very effective inhibitor of thepolymerization of isoprene and acrylonitrile. D1 additionally describeshow alpha-tocopherol and hydroquinone have a synergistic, i.e.,superadditive, effect in the inhibition of the polymerization ofacrylonitrile. In order to determine the effectiveness of the inhibitionof polymerization, an induction time was measured in D1. This is thetime up to which, under defined conditions, it was possible to suppresspolymerization of the monomer by adding the inhibitors.

[0015] A further text relating to the stabilization of monomers is D2.In this document an improved process is described for preparing(meth)acrylic esters of polyhydric alcohols. In this case, stabilizationwith tocopherol is disclosed in the acidic esterification of methacrylicacid with alcohols. Furthermore, D2 states that, for the preparation ofradiation-curable (meth)acrylic esters with high purity and inparticular a low intrinsic color, three types of stabilizers areaccorded particular importance. These are hydroquinone itself,di-tert-butylhydroquinone, and sterically hindered phenol compounds ofthe tocopherol type, especially alpha-tocopherol. According to D2 theuse of di-tert-butylhydroquinone in the preparation(transesterification) leads without problems to pale-coloredstorage-stable (meth)acrylic esters. The use of tocopherols as wellleads with comparatively few problems to pale-colored products of thedesired kind. If instances of color deterioration arise duringpreparation, according to D2, they can be eliminated without difficultyby aftertreatment with alumina. Information on the color stabilizationof monomers during storage (i.e., after preparation) cannot be inferredfrom D2.

[0016] The following publications relate to the stabilization ofpolymers.

[0017] D3 relates to the prevention of the discoloration (“browning”) ofpolymers. It discloses a composition for coating materials or binderswhich comprises a copolymer of dimethylaminomethyl methacrylate andmethyl methacrylate units and also tocopherol. The “browning” of thecopolymer, which is readily soluble in organic solvents and water, isprevented by the addition of tocopherol.

[0018] D4 relates to the stabilizing of polymers (organic materials),particularly polyolefins, against thermal, oxidative or light-induceddegradation, with a stabilizer mixture comprising at least one monomericor oligomeric organic biphosphite together with at least onebenzofuran-2-one or alpha-tocopherol (vitamin E). It is evident,however, in the examples of D4 (Example 2a) that in the case of theaddition of benzofuran-2-one compound, which is understood as analternative to tocopherol with equivalent activity, an increase in theyellowness index is observed, which in turn is an indicator that thereis serious discoloration of the stabilized polymer material. Animprovement in the color stabilization during extrusion can clearly beattributed here to the organic phosphite compound (Example 2b).

[0019] A further prior art document which concerns itself with thestabilization of polymeric material is D5. In this publication thestabilization of polymers of styrene or of polymers of a (meth)acryliccompound against degradation by light and/or heat and/or atmosphericoxygen, using a mixture of an organic phosphite and alpha-tocopherol isdescribed. Concerning the color stability of the stabilized moldingcompound during storage, however, nothing is said.

[0020] In the light of the state indicated and discussed herein it wasan object of the invention to find a possibility for effectivestabilization of the color of ethylenically unsaturated monomers,especially hydroxyalkyl (meth)acrylates, which already have a base-levelstabilization. A particular intention was to prevent the occurrence ofdiscolorations during storage, which have been found in practice againand again in ethylenically unsaturated monomers stabilized adequatelyper se against polymerization.

[0021] These objects and also further objects which, although not statedexplicitly, can nevertheless be readily inferred from the introductorydiscussion of the state of the art or derived therefrom as self-evidentare achieved by means of a use with all of the features of theindependent main claim.

[0022] Advantageous embodiments of the use according to the inventionare subject matter of the claims which relate back to the independentuse claim.

[0023] As a result of using at least one compound from the group of thetocopherols for the color stabilization of ethylenically unsaturatedmonomers, in particular of hydroxyalkyl (meth)acrylates, which alreadyhave at least one polymerization inhibitor for the purpose of base-levelstabilization or storage stabilization, it is possible in a not readilyforeseeable way to prevent the discoloration of monomers during storage.

[0024] Surprisingly it has been found that the addition of tocopherol toa conventionally stabilized monomer solution raises the color stabilityin the case of vinyl monomers, i.e., its addition in the ppm rangecounters the discoloration of monomers. With a series of experiments itwas possible to show that the color number with a given stabilizermixture increases continuously without tocopherol, whereas withtocopherol no increase in color, or only a moderate increase in color,was observed.

[0025] In the context of the invention a tocopherol compound is used tostabilize the color of ethylenically unsaturated monomers.

[0026] The tocopherol compounds which can be used for the purposes ofthe invention are chroman-6-ols (3,4-dihydro-2H-1-benzopyran-6-ols)substituted in position 2 by a 4,8,12-trimethyltridecyl radical. Thetocopherols which can be used with preference in accordance with theinvention include alpha-tocopherol, beta-tocopheral, gamma-tocopherol,delta-tocopherol, zeta2-tocopherol, and eta-tocopherol, all of theaforementioned compounds in each case in the (2R,4′R,8′R) form, andalpha-tocopherol in the (all-rac) form. Preference is given toalpha-tocopherol in the (2R,4′R,8′R) form (trivial name:RRR-alpha-tocopheral) and to the synthetic racemic alpha-tocopherol(all-rac-alpha-tocopherol). Of these the last-mentioned one in turn isof particular interest in view of the relatively low price.

[0027] The amount of tocopherol compound which can be employed tostabilize the color of base-level-stabilized monomers can differ over awide range depending on monomer and desired efficiency. For many fieldsof use amounts of up to 1000 ppm, based on the sum of the weights ofmonomer and tocopherol compound are sufficient. In many cases even verysmall added amounts of 10 ppm are enough to obtain a perceptibleimprovement in the color stability. Should less than 10 ppm be used,however, a substantial color stabilization is generally imperceptible. Afavorable range for the amount to be added is therefore between 10 and1000 ppm of tocopherol compound based on the sum of the weights ofmonomer and tocopherol compound. Very good results are achieved in theaddition range from 100 to 800 ppm. One particular embodiment of theinvention provides for use in an amount of from 200 to 600 ppm. It isespecially appropriate to use about 400 ppm of tocopherol compound.

[0028] In connection with the invention ethylenically unsaturatedmonomers are compounds which contain at least one free-radicallypolymerizable double bond. These ethylenically unsaturated monomersinclude vinyl esters, (meth)acrylic acid, esters of (meth)acrylic acid,for example, methyl and ethyl (meth)acrylate, vinyl chloride, vinylidenechloride, vinyl acetate, styrene, substituted styrenes having an alkylsubstituent in the side chain, such as α-methylstyrene andα-ethylstyrene, substituted styrenes having an alkyl substituent on thering, such as vinyltoluene and p-methylstyrene, halogenated styrenes,such as monochlorostyrene, dichlorostyrenes, tribromostyrenes, andtetrabromostyrenes, vinyl and isopropenyl ethers, maleic acidderivatives, such as maleic anhydride, methylmaleic anhydride,maleimide, and methylmaleimide, and dienes, such as divinylbenzene, forexample.

[0029] The (meth)acryl-notation embraces methacryl-, acryl-, andmixtures of both.

[0030] Preferred ethylenically unsaturated monomers are (meth)acrylicacid and also derivatives of (meth)acrylic acid. These compounds can berepresented by way of example in accordance with formula 1

[0031] in which the radical R¹ is hydrogen or a methyl group, theradical R² is hydrogen, an aryl radical, which may also containheteroatoms, such as phenyl and imidazole, for example, and also astraight-chain, branched or cyclic alkyl radical having up to 30 carbonatoms, which may be either saturated or unsaturated, and may alsocontain heteroatoms, such as nitrogen and/or oxygen, for example, suchas methyl, ethyl, propyl, butyl, pentyl, hexyl, cyclopentyl, cyclohexyl,isobornyl, vinyl, propenyl, butynyl, 2-(N,N-di-methylamino)ethyl,2-(N,N-dimethylamino)propyl, 2-hydroxypropyl, and 2-hydroxyethyl, forexample.

[0032] These compounds include methyl (meth)acrylate, ethyl(meth)acrylate, propy (meth)acrylate, isoprpyl (meth)acrylate, n-butyl(meth)acrylate, isobornyl (meth)acrylate, hydroxyalkyl (meth)acrylates,such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate,3-hydroxypropyl (meth)acrylate, and 3,4-dihydroxybutyl (meth)acrylate,and also aminoalkyl (meth)acrylates, such as dimethylaminoethylmethacrylate (DMAEMA).

[0033] The (meth)acrylamides corresponding to the (meth)acrylic estersare likewise a preferred group of ethylenically unsaturated monomers.They include for example N-dimethylaminopropylmethacrylamide (DMAPMA).

[0034] The ethylenically unsaturated monomers can be presentindividually or as a mixture in the composition of the invention.

[0035] One especially preferred group of compounds whose color isstabilized in accordance with the principle of the invention are thehydroxyalkyl (meth)acrylates.

[0036] Of particular interest in this context is the use for colorstabilization of a compound selected from the group consisting ofhydroxyethyl acrylate (HEA), hydroxyethyl methacrylate (HEMA),2-hydroxypropyl acrylate (2-HPA), 2-hydroxypropyl methacrylate (2-HPMA),3-hydroxypropyl acrylate (3-HPA), 3-hydroxypropyl methacrylate (3-HPMA),2,3-dihydroxypropyl acrylate, 2,3-dihydroxypropyl methacrylate,1,3-diacryloylglycerol, 1,3-dimethacryloylglycerol, trimethylolpropanemonoacrylate, trimethylolpropane monomethacrylate, trimethylolpropanediacrylate, and trimethylolpropane dimethacrylate.

[0037] Esterification products of (meth)acrylic acid with polyhydricalcohols can also in particular be stabilized durably and effectively interms of color through the inventive use of tocopherol.

[0038] Esterification products whose stabilization is particularlypreferred include the (meth)acrylic esters of polyalcohols from thegroup embracing the following: ethylene glycol, propylene glycol,butane-1,4-diol, hexane-1,6-diol, neopentylglycol, diethylene glycol,triethylene glycol, dimethylolpropane, glycerol, trimethylolpropane,trimethylolhexane, trimethylolethane, hexane-1,3,5-triol, andpentaerythritol.

[0039] The monomers to be stabilized with respect to discoloration inaccordance with the invention already include at least one stabilizingcompound for the purpose of base-level stabilization. This compound isused to inhibit polymerization.

[0040] Polymerization inhibitors are already known. For example,1,4-dihydroxybenzenes can be added for stabilization. It is, however,also possible to employ differently substituted dihydroxybenzenes. Ingeneral terms such inhibitors can be represented by the general formula(II)

[0041] in which

[0042] R¹ is hydrogen, a linear or branched alkyl radical having one toeight carbon atoms, halogen or aryl, preferably an alkyl radical havingone to four carbon atoms, more preferably methyl, ethyl, n-propyl,iso-propyl, n-butyl, iso-butyl, sec-butyl, tert-butyl, Cl, F or Br;

[0043] n is an integer in the range, from one to four, preferably one ortwo; and

[0044] R² is hydrogen, a linear or branched alkyl radical having one toeight carbon atoms or aryl, preferably an alkyl radical having one tofour carbon atoms, more preferably methyl, ethyl, n-propyl, iso-propyl,n-butyl, iso-butyl, sec-butyl or tert-butyl.

[0045] It is, however, also possible to use compounds with1,4-benzoquinone as parent compound. These can be described by theformula (III)

[0046] in which

[0047] R¹ is a linear or branched alkyl radical having one to eightcarbon atoms, halogen or aryl, preferably an alkyl radical having one tofour carbon atoms, more preferably methyl, ethyl, n-propyl, iso-propyl,n-butyl, iso-butyl, sec-butyl, tert-butyl, Cl, F or Br; and

[0048] n is an integer in the range from one to four, preferably one ortwo.

[0049] Use is also made of phenols of the general structure (IV).

[0050] in which

[0051] R¹ is a linear or branched alkyl radical having one to eightcarbon atoms, aryl or aralkyl, proprionic esters with 1 to 4 valentalcohols, which may also contain heteroatoms such as S, and N,preferably an alkyl radical having one to four carbon atoms, morepreferably methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl,sec-butyl, tert-butyl.

[0052] A further advantageous class of substance is represented bysterically hindered phenols based on triazine derivatives of the formula(V):

[0053] with R=compound of the formula (VI)

[0054] in which

[0055] R¹=C_(n)H_(2n+1)

[0056] with n=1 or 2.

[0057] A further group of known inhibitors are amines, especiallysterically hindered amines.

[0058] These include, in particular, phenylenediamines, which can berepresented by formula (VII)

[0059] in which R¹, R², R³, and R⁴ independently are hydrogen and alsoalkyl, aryl, alkaryl, and aralkyl radicals having in each case up to 40,preferably up to 20 carbon atoms, preferably at least one of theradicals R¹, R², R³, R⁴ being hydrogen.

[0060] Exemplary p-phenylenediamines include p-phenylenediamine in whichthe radicals R¹, R², R³, and R⁴ are hydrogen;

[0061] N-phenyl-N′-alkyl-p-phenylenediamines, for example,N-phenyl-N′-methyl-p-phenylenediamine,N-phenyl-N′-ethyl-p-phenylenediamine,N-phenyl-N′-propyl-p-phenylenediamine,N-phenyl-N′-isopropyl-p-phenylenediamine,N-phenyl-N′-n-butyl-p-phenylenediamines,N-phenyl-N′-isobutyl-p-phenylenediamine,N-phenyl-N′-sec-butyl-p-phenylenediamine,N-phenyl-N′-tert-butyl-p-phenylenediamine,N-phenyl-N′-n-pentyl-p-phenylenediamine,N-phenyl-N′-n-hexyl-p-phenylenediamine,N-phenyl-N′-(1-methylhexyl)-p-phenylenediamine,N-phenyl-N′(1,3-dimethylbutyl)-p-phenylenediamine,N-phenyl-N′(1,4-dimethylpentyl)-p-phenylenediamine;

[0062] N-phenyl-N′,N′-dialkyl-p-phenylenediamines, such asN-phenyl-N′,N′-dimethyl-p-phenylenediamine,N-phenyl-N′,N′-diethyl-p-phenylenediamine,N-phenyl-N′,N′-di-n-butyl-p-phenylenediamine,N-phenyl-N′,N′-di-sec-butyl-p-phenylenediamine,N-phenyl-N′-methyl-N′-ethyl-p-phenylenediamine, for example;

[0063] N,N-dialkyl-p-phenylenediamines, such asN,N-dimethyl-p-phenylenediamine and N,N′-diethyl-p-phenylenediamine, forexample;

[0064] N,N′-dialkyl-p-phenylenediamines, such asN,N′-diisopropyl-p-phenylenediamine, N,N′-diisobutyl-p-phenylenediamine,for example;

[0065] N,N′-diarylphenylenediamines, such asN,N′-diphenyl-p-phenylenediamine, for example;

[0066] N,N,N′-trialkyl-p-phenylenediamines, such asN,N,N′-trimethyl-p-phenylenediamine, N,N,N′-triethyl-p-phenylenediamine,for example.

[0067] Over and above these, phenazine dyes form a further preferredgroup. These include, in particular, induline and nigrosine. Nigrosineis formed by heating nitrobenzene, aniline, and hydrochloric anilinewith metallic iron and FeCl₃. Preference is given in this context toalcohol-soluble aniline dyes, which may include, for example, 5 benzenenuclei, such as dianilido-N,N-diphenylphenosafranin. These substancesare widely known and can be obtained commercially.

[0068] Particularly successful are the compounds 1,4-dihydroxybenzene,4-methoxyphenol, 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone,1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxybenzyl)benzene,2,6-di-tert-butyl-4-methylphenol, 2,4-dimethyl-6-tert-butyl-phenol,2,2-bis[3,5-bis(1,1-dimethylethyl)-4-hydroxy-phenyl-1-oxopropoxymethyl)]1,3-propanediylester, 2,2′-thiodiethylbis[3-(3,5-di-tert-butyl-4-hydroxy-phenyl)]propionate,octadecyl3-(3,5-di-tert-butyl-4-hydroxyphenyl-propionate,3,5-bis(1,1-dimethylethyl-2,2-methylenebis(4-methyl-6-tert-butyl)phenol,tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)-s-triazine-2,4,6-(1H,3H,5H)trione,tris(3,5-di-tert-butyl-4-hydroxy)-s-triazine-2,4,6-(1H,3H,5H)trione,tert-butyl-3,5-dihydroxybenzene or diphenyl-p-phenylenediamine (DPPD),of which, in turn, hydroquinone monomethyl ether (4-methoxyphenol) isespecially appropriate.

[0069] The stated inhibitors are available commercially.

[0070] As base-level stabilization for ethylenically unsaturatedcompounds it is possible to employ the stated compounds alone or in amixture of two or more compounds.

[0071] Based on the weight of the overall composition the fraction ofthe inhibitors responsible for the base-level stabilization of themonomers, individually or as a mixture, is for many applications0.01.-0.5% (wt/wt).

[0072] For certain applications it can be of particular advantage tocarry out base-level stabilization with a mixture ofN,N-diethylhydroxylamine and N-nitroso-N-phenylhydroxylamine(“cupferron”). N-Nitroso-N-phenyl-hydroxylamine is an acidic compound,and its salts, such as the ammonium, aluminum, copper, lithium, sodium,potassium, and rubidium salt, for example, can likewise be used.

[0073] The combination of the compounds N,N-diethylhydroxylamine andN-nitroso-N-phenylhydroxylamine can lead to a synergistic effect in thecontext of the base-level stabilization.

[0074] Synergism here means that the inhibitory effect of thecombination is greater than the inhibition that can be expected on thebasis of the activity of the individual compounds.

[0075] In the context of use in accordance with the invention the ratioof color stabilizer to base-level stabilizer is variable. For manymonomers it is appropriate to use them in a ratio (w/w) of compound fromthe group of the tocopherols to polymerization inhibitor (base-levelstabilizer) of from 1:10 to 10:1, preferably from 1:1 to 8:1.

[0076] The inventively stabilized monomer composition may comprisefurther constituents. These include solvents, such as benzene, toluene,n-hexane, cyclohexane, methyl isobutyl ketone, and methyl ethyl ketone,for example.

[0077] It is additionally possible for known additives, such asantibinding agents, antistats, antioxidants, biostabilizers, chemicalblowing agents, mold release agents, flame retardants, lubricants,colorants, pour improvers, fillers, glidants, adhesion promoters,catalysts, light stabilizers, optical brighteners, organophosphoruscompounds, oils, pigments, impact modifiers, reinforcing agents,reinforcing fibers, weathering protectants, and plasticizers to bepreferred components of the monomer composition stabilized incorrespondence with the use in accordance with the invention.

[0078] The compositions stabilized by use in accordance with theinvention can be obtained by mixing at least one ethylenicallyunsaturated monomer and an effective stabilizing amount of a base-levelstabilizer and also at least one tocopherol compound.

[0079] Vinyl monomers, especially HEMA, find use in clearcoat andhigh-solid coating materials. The use of colorless starting materials inthis context has particularly favorable consequences for the color ofthe end product. The use of certain hydroxyalkyl (meth)acrylates in theabovementioned applications is already described in EP 0882750 and U.S.Pat. No. 5,753,756.

[0080] The inventive and comparative examples below serve for furtherillustration of the present invention, without any restriction beingintended thereby.

[0081] I. Methods

[0082] I.1. General Experimental Procedure

[0083] The experiments were conducted by adding additives to fullyformulated monomers which in order to inhibit polymerization possessed abase-level stabilization which is indicated in the examples(stabilization with 200 ppm of hydroquinone monomethyl ether (HQME)),with the concentrations indicated in the table. The samples weresubjected to a storage test in comparison to the corresponding blanksamples at 50° C. In the course of this test, at certain intervals thePt/Co color number was determined along the lines of DIN/ISO 6271 as afunction of the time. In addition, nephelometry was used to determinethe turbidity of the samples, so as to investigate the inhibition ofpolymerization.

[0084] I.2. Determination of the Turbidity of Liquid Organic Compounds(Nephelometry)

[0085] I.2.1. Principle

[0086] A light beam is passed through the sample. Photoelectric cellsare used to measure the light scattered at an angle of 90°, theforward-scattered light, and the transmitted light. The ratio of theoutput of the photoelectric cell for the 90° measurement to the sum ofthe outputs of the other two photoelectric cells is formedelectronically and in this way outstanding linearity is achieved and theinfluence of the color is excluded. As a result of the construction ofthe optical system the scattered light can be disregarded. Scatteredlight in the context of nephelometric measurements is the portion of thelight which reaches the photoelement but cannot be attributed to lightscattering in the sample. The samples under analysis are measured usinga turbidity-measuring instrument. The HACH* Ratio turbidity meter is alaboratory nephelometer which allows turbidities of up to 199 turbidityunits (NTU) to be measured on the basis of formazine even in coloredsamples. Solutions with a higher turbidity can also be measuredfollowing dilution of the sample, with a filtered sample solution, andon the basis of a simple calculation.

[0087] I.2.2. Technical Data of the Ratio Turbidity Meter Calibrationstandard: formazine Sample size: 30 ml Cuvette: 25 × 95 mm; glassbottles with screw cap Operating temperature range: +10° to +45° C.Reaction time: less than 15 seconds until equilibrium is reached in thegreatest measuring range Reproducibility: 1% of the measuring range plus±1 at the smallest point Accuracy: ±2% of the measuring range plus ±1 atthe smallest point Measuring range: 0-200 NTU

[0088] I.2.3. Sample Preparation

[0089] a. Switch on instrument and wait for about 15 minutes for it towarm up;

[0090] b. Switch on measuring area and insert the correspondingturbidity standard and again wait for about 15 minutes;

[0091] c. The indicator must be at ±5% of the nominal value (otherwisecalibration must be repeated);

[0092] d. Fill a clean cuvette up to the mark with test liquid andadjust;

[0093] e. The turbidity of the sample appears on the digital display

[0094] I.2.4. Meaning and Significance of the Measurements <2 NTUinvisible to the eye 2 to 5 NTU just visible to the eye >5 NTU distinctturbidity >20 NTU severe turbidity

[0095] I.3. Photometric Determination of the Platinum-cobalt ColorNumber of Monomers

[0096] I.3.1. Principle

[0097] The visual comparison with color standard solutions of theplatinum-cobalt scale is replaced by a measurement of the absorbance ofthe sample at the wavelengths of 460 and 620 nm. The difference inabsorbance A_(460 nm)−A_(620 n)=ΔA stands in a linear relation to thecolor unit of the platinum-cobalt standards. When the color number isplotted as a function of ΔA a calibration plot is obtained whose slopeserves directly as the “factor” for the calculation of the color number.The assumption is that the sample under analysis corresponds to thecolor characteristic, i.e. corresponds substantially in shade to theplatinum/cobalt scale. Synonyms for the platinum-cobalt color number areAPHA number and Hazen number.

[0098] I.3.2. Procedure

[0099] I.3.2.1. Apparatus

[0100] Spectrophotometer or filter photometer having filters for theranges 460 and 620 nm; 5 cm and 1 cm cuvettes Balance (d=1 mg);volumetric flask; volumetric cylinder

[0101] I.3.2.2. Calibrating Substances

[0102] Potassium hexachloroplatinate (IV) (K₂PtCl₆)

[0103] Cobalt(II) chloride hexahydrate (COCl₂×6H₂O)

[0104] Conc. hydrochloric acid p.a. 32%

[0105] I.3.2.3. Establishment of the Calibration Function

[0106] I.3.2.3.1 Preparation of the Color Standard

[0107] 1.245 g of hexachloroplatinate(IV) and 1.000 g of cobalt(II)chloride hexahydrate are dissolved in DI water; 118.8 ml of the conc.hydrochloric acid are added and the solution is made up to the mark withDI water in a 1000 ml volumetric flask. This solution possesses a Pt/Cocolor number of 500 provided that it corresponds to the limiting valuesindicated in the compilation below for the transmittance and absorbancewhen measured with the spectrophotometer in a 1 cm cuvette against DIwater. Wavelength (nm) Transmittance Absorbance 430 0.759 to 0.776 0.110to 0.120 455 0.716 to 0.741 0.130 to 0.145 480 0.759 to 0.785 0.105 to0.120 510 0.861 to 0.881 0.055 to 0.065

[0108] I.3.2.3.2. Preparation and Measurement of the Pt/Co StandardComparison Solutions

[0109] Pt/Co standard comparison solutions for the required measurementrange (0-500 of the color scale) can be prepared from the Pt/Co stocksolution by introducing corresponding volumes of standard solution into100 ml volumetric flasks which are made up to the mark with DI water.The volumes of stock solution for the desired Pt-Co standard comparativesolutions can be taken from the tabular compilation below:

[0110] Pt-Co standard comparison solutions Volume of the stock Colorsolution Pt—Co color number ml 0 0 5 1 10 2 15 3 20 4 30 6 40 8 50 10 7014 100 20 150 30 200 40 300 60 400 80 500 100

[0111] The Pt-Co standard comparison solutions prepared are measured in5 cm cuvettes with the spectrophotometer or with a filter photometerwith the corresponding filters at 460 and 620 nm. (Reference cuvettecontains DI water). The Pt/Co color numbers and the associateddifferences in absorbance that are measured (A460 nm-A620 nm) produce alinear relationship. The slope of this calibration plot can bedetermined graphically or, better still, by regression calculation andserves as a basis for calculating the Pt/Co color number (=factor).

[0112] I.3.2.4. Assessing the Samples and Carrying Out PhotometricMeasurement

[0113] I.3.2.4.1. Visual Assessment

[0114] Prior to the actual measurement it is necessary to carry out avisual examination as to whether the sample corresponds to the colorcharacteristic (yellow hue, for example, by comparison with the standardcomparison solutions) of the Pt/Co color scale or deviates therefrom. Ifthe latter is the case, then no measurement takes place. If the sampleexhibits a turbidity, then stating the color number in accordance withthe underlying color scale here makes no sense. In the case of uncoloredsamples, turbidities can simulate color numbers >100.

[0115] I.3.2.4.2. Photometric Measurement

[0116] If the color characteristic matches, the liquid for measurementis introduced into a 5 cm cuvette and the cuvette is sealed. It must befree from air bubbles or streaks. Then the absorbance of the sample ismeasured with a spectrophotometer or with a filter photometer with thecorresponding filters at 460 and 620 nm against DI water and thedifference in absorbance is calculated.

[0117] I.3.2.5. Calculation of the Pt/Co Color Number (APHA Number)

Pt/Co color number=(A ₄₆₀ nm−A ₆₂₀ nm)×factor (5 cm)

[0118] Factor (varian cary 1 spectrophotometer)=755.2

[0119] Since the factor may adopt different values depending on thespecific instrument, it must be determined in each case in accordancewith 1.3.3.3. If absorbances <0 occur at 620 nm, then again thedifference is formed; in other words, the absorbance figure at 460 nmhas the numerical value of the absorbance at 620 nm added to it.

[0120] I.3.2.6. Results Reporting

[0121] If the sample is more transmittant than DI water, i.e.,absorbances of between 0 and negative values are obtained at 460 and 620nm, then fundamentally <5 is reported.

[0122] Where the calculated color number is between 0 and 5, <5 isreported.

[0123] Where the calculated color number is between 5 and 200, theresult is reported rounded to whole numbers.

[0124] When the calculated color number is between 200 and 500, theresult is rounded to 5 units.

[0125] I.4. Method of Determining the Polymerization Time (PTMeasurement)

[0126] I.4.1. Thermal Polymerization in an Oil Bath

[0127] A test tube (18×180 mm) is filled with 20 g of the test mixture.With the aid of a specially manufactured, drilled ®Plexidur stopper aglass tube (diameter 7 mm) which has been closed by melting at one endis centered in the test tube. The length of the melt-closed glass tubeis approximately 75 mm. It ends about 1.8 cm above the base of the testtube. A small amount of a high-boiling plasticizer is introduced intothis glass tube, as a heat transfer medium, and an Fe/constantanthermocouple is inserted. At the beginning of the polymerization timemeasurement the test tube containing the test mixture is inserted into acirculation bath thermostat which has been preheated to polymerizationtemperature. The depth of immersion is adjusted reproducibly by means ofa plastic mesh. The level of the oil bath is at least 1 cm above thelevel of the liquid in the test tube. Simultaneously with the insertionof the test tube into the oil bath a PC-supported temperature measuringposition with six measuring points is switched on. The temperature/timeplot is recorded and evaluated. After the polymerization process issubstantially at an end, the measurement is terminated.

[0128] I.4.2

[0129] The measured values obtained and recorded are, in dependence onthe measuring point, the time taken to achieve the temperature maximumand the peak polymerization temperature.

[0130] II. Results

[0131] II.1. HEMA—Color Study on Two Batches (A and B) Statement of thePt-Co Color Number in Accordance with I.3.3.

[0132] The results are summarized as follows:

[0133] 30° C. storage Batch A Batch B +400 ppm +400 ppm Batch Atocopherol Batch B tocopherol Beginning of study <5 <5 <5 <5 after 2weeks 10 8 7 8 after 4 weeks 8 5 5 6 after 6 weeks 13 10 10 10 after 8weeks 14 9 11 9 after 10 weeks 15 9 13 9

[0134] 50° C. storage Batch A Batch B +400 ppm +400 ppm Batch Atocopherol Batch B tocopherol Beginning of study <5 <5 <5 <5 after 2weeks 21 11 22 10 after 4 weeks 22 8 18 9 after 6 weeks 25 12 20 11after 8 weeks 24 13 19 13 after 10 weeks 23 13 19 12

[0135] II.2. HEMA—Polymer Test on Stored Samples

[0136] Turbidity measurement in accordance with I.2., 1:9 in diethylether.

[0137] Statement of the NTU values.

[0138] After 10 weeks at 50° C. Additive Turbidity Batch A 0.35 NTUBatch A 400 ppm of tocopherol 0.30 NTU Batch B 0.30 NTU Batch B 400 ppmof tocopherol 0.29 NTU

[0139] The turbidity values obtained unambiguously permit the conclusionthat the samples contain no polymer fractions at all. Values >200 NTUwere obtained on incipiently polymerized HEMA (0.05% initiator. 80° C.,approx. 50 min) in the turbidity test.

[0140] The improvement in the turbidity value through the addition oftocopherol is only minimal. This means that the addition of tocopherolmakes no significant contribution to the stabilization beforepolymerization.

[0141] In order to confirm this result a further investigation was setup.

[0142] II.3. Determination of the Polymerization Times (PT Test)

[0143] At the processing temperatures typical for the applications(e.g., in coating materials) the polymerization time of samples withbase-level stabilization by HQME were investigated and were comparedwith the polymerization time of samples additionally containing anamount of alpha-tocopherol effective for the purpose of colorstabilization.

[0144] Conditions

[0145] Comparison sample:

[0146] Hydroxyethyl methacrylate (HEMA) stabilized with 200 ppm ofhydroquinone methyl ether (HQME);

[0147] Inventive sample:

[0148] Hydroxyethyl methacrylate (HEMA) stabilized with 200 ppm ofhydroquinone methyl ether (HQME) and additionally containing 400 ppm oftocopherol;

[0149] Polymerization temperature: 110° C.;

[0150] Initiator: 1.0% (w/w) tert-butyl peroctoate;

[0151] Comparison sample:

[0152] Polymerization time: 2.8 min;

[0153] Temperature maximum: 175.5° C.;

[0154] Sample in acc. with invention;

[0155] Polymerization time: 3.0 min;

[0156] Temperature maximum: 168.8° C.

[0157] These values also demonstrate that the addition of tocopherol hasno significant influence on the inhibition of polymerization. In spiteof this, and in a manner which was not immediately foreseeable, it isthe case, however, that the color of base-level-stabilized monomers isstabilized over time.

1. The use of at least one compound from the group of the tocopherolsfor the color stabilization of ethylenically unsaturated monomers, inparticular of hydroxyalkyl (meth)acrylates, which already have at leastone polymerization inhibitor for base-level stabilization or storagestabilization.
 2. The use as in claim 1 of all-rac-alpha-tocopherol. 3.The use as in one of the preceding claims in an amount of from 10 to1000 ppm based on the sum of the weights of tocopherol compound andethylenically unsaturated monomer.
 4. The use as in claim 3 in an amountof from 100 to 800 ppm based on the sum of the weights of tocopherolcompound and ethylenically unsaturated monomer.
 5. The use as in claim 4in an amount of from 200 to 600 ppm based on the sum of the weights oftocopherol compound and ethylenically unsaturated monomer.
 6. The use asin one of the preceding claims for the color stabilization ofhydroxyalkyl (meth)acrylates.
 7. The use as in claim 6 for the colorstabilization of a compound selected from the group consisting ofhydroxyethyl acrylate (HEA), hydroxyethyl methacrylate (HEMA),2-hydroxypropyl acrylate (2-HPA), 2-hydroxypropyl methacrylate (2-HPMA),3-hydroxypropyl acrylate (3-HPA), 3-hydroxypropyl methacrylate (3-HPMA),2,3-dihydroxypropyl acrylate, 2,3-dihydroxypropyl methacrylate,1,3-diacryloylglycerol, 1,3-dimethlacryloylglycerol, trimethylolpropanemonoacrylate, trimethylolpropane monomethacrylate, trimethylolpropanediacrylate, and trimethylolpropane dimethacrylate.
 8. The use as in oneof the preceding claims, wherein the polymerization inhibitor, orbase-level stabilizer is a hydroquinone, preferably hydroquinonemonomethyl ether.
 9. The use as in one of the preceding claims in aratio (w/w) of compound from the group of the tocopherols topolymerization inhibitor of from 1:10 to 10:1, preferably from 1:1 to8:1.
 10. The use as in one of the preceding claims of color-stabilizedand polymerization-inhibited ethylenically unsaturated monomers,preferably hydroxyalkyl (methacrylates, in clearcoat materials orhigh-solids coating materials.